This project is concerned with studies of the genetic diversity of Trypanosoma cruzi and the implications of this diversity in the presentation and course of Chagas' disease. Major emphasis during the course of the year has centered on four topics: 1) A continuation of the multivariant analysis of T. cruzi clones; 2) An analysis of the surface proteins of T. cruzi clones; 3) The development and testing of an ultra-high resolution flow cytometer; 4) A histochemical characterization of skeletal muscle fibers of mice infected with T. cruzi. The multivariant analysis has progressed to the point where it can be demonstrated that several variables such as total DNA/cell, growth rate, allopurinol riboside sensitivity or resistance, etc. are highly associated. The virulence of a T. cruzi clone is related to this association. Consequently, these results support the concept that parasite genetic heterogeneity plays an important role in the presentation and course of Chagas' disease. A high resolution flow cytometer incorporating many features not found in commercially available instruments has become fully operational after a year of intensive development and testing. The unique capabilities of this instrument should prove useful in the analysis of numerous biochemical and physiological parameters of medically important protozoa. The surface components of 4 T. cruzi clones have been studied extensively. Major and minor proteins are shared by all 4 clones. However, several clone-specific proteins were also detected. The leg muscles of mice infected with T. cruzi were typed histochemically. Although markedly more type II fibers were present, nearly 5-fold more type I fibers were infected than type II. This is the first demonstration of a common denominator to explain the in vivo distribution of T. cruzi in muscle fibers.